Nucleases are useful reagents for removing nucleic acids from protein preparations. All cells contain nucleases. Some nucleases can degrade both DNA as well as RNA in their double- and single-stranded forms. Some of nucleases degrade either DNA or RNA. Some of nucleases have more specific substrates, for example, single-stranded DNA. Reagent nucleases have been isolated either from the native host or as recombinant proteins. A problem encountered with cloning nucleases, which cleave both DNA and RNA, is that these proteins become toxic to host cells such as E. coli if overexpression is attempted using E. coli or other hosts. The toxicity effect can be ameliorated to some extent by causing the host cell either to efficiently secret the nuclease or to sequester the nuclease in an inclusion body in the host cell. Both methods have some disadvantages. Obtaining purified nuclease from media requires handling large volumes, which can be laborious. Release of nucleases from inclusion bodies in lysed cells requires denaturing quantities of urea (for example 6M urea) after which the nuclease has to be renatured to restore its enzymatic activity. It would therefore be desirable to have methods of obtaining nucleases in a simple way.
Descriptions of certain nucleases and their preparation by cloning are provided by Eaves et al. (J. Bacteriol. 85:273-8 (1963)), Filimonova et al. (Biochem. Mol. Biol. Int. 33(6):1229-36 (1994)), Ball et.al. (Gene 57(2-3):183-92 (1987), Molin et. al. (U.S. Pat. No. 5,173,418), and Friedhoff et al. (Protein Expr. Purif. 5(1):37-43 (1994)).